Jupiter may be the solar system’s oldest planet
AMY A. SIMON, NASA, EUROPEAN SPACE AGENCY
Jupiter was an early bloomer. A close look at the ages of rock and metal fragments from the birth of the solar system suggests the giant planet formed early on. Probably within the solar system’s first million years. If so, Jupiter’s presence could help explain why the inner planets are so small. It may even be responsible for Earth’s existence, a new study suggests.
Previously, astronomers estimated Jupiter’s age with computer models. These simulations show how solar systems form in general. Gas giants like Jupiter grow by piling on more and more gas. This gas comes from spinning disks of gas and dust around a young star. The disks typically don’t last more than 10 million years. So astronomers inferred that Jupiter formed by the time that the sun’s disk disappeared. It had to have been born at least 10 million years after the solar system started to form.
“Now we can use actual data from the solar system to show Jupiter formed even earlier,” says Thomas Kruijer. He is a geochemist. He studies the chemical composition of rocks. Kruijer did the research while at the University of Münster in Germany. He is now at Lawrence Livermore National Laboratory in California. To study Jupiter, one of the biggest objects in the solar system, he and colleagues turned to some of the smallest: meteorites.
Meteorites are lumps of material from space that land on Earth. Most meteorites come from the asteroid belt. This is a ring of rock currently located between Mars and Jupiter. But those lumps of rock and metal were probably born elsewhere.
Luckily, meteorites carry a signature of their birthplaces. The gas and dust disk that the planets formed from contained different neighborhoods. Each had the equivalent of its own “zip code.” Each is enriched in certain isotopes. Isotopes are atoms of the same element that have different masses. Careful measurements of a meteorite’s isotopes can point to its birthplace.
Kruijer and colleagues selected 19 samples of rare iron meteorites. The samples came from the Natural History Museum in London, England, and the Field Museum in Chicago, Ill. These rocks represent the metal cores of the first asteroid-like bodies to congeal as the solar system was forming.
The team put a gram of each sample into a solution of nitric acid and hydrochloric acid. Then, the researchers let it dissolve. “It smells terrible,” Kruijer says.
They then separated out the element tungsten. It is a good tracer of both a meteorite’s age and birthplace. They also took out the element molybdenum. It is another tracer of a meteorite’s home.
The team looked at the relative amounts of certain isotopes of the elements: molybdenum-94, molybdenum-95, tungsten-182 and tungsten-183. From the data, the team identified two distinct groups of meteorites. One group formed closer to the sun than Jupiter is today. The other formed farther from the sun.
The tungsten isotopes also showed that both groups existed at the same time. The groups existed between about 1 million and 4 million years after the start of the solar system. The solar system was born about 4.57 billion years ago. That means something must have kept the two groups separated.
The most likely candidate is Jupiter, Kruijer says. His team calculated that Jupiter’s core had probably grown to about 20 times the mass of the Earth in the solar system’s first million years. That would make Jupiter the oldest planet in the solar system. Its early existence would have created a gravitational barrier: That barrier would have kept the two rock neighborhoods segregated. Jupiter would then have continued growing at a slower rate for the next few billion years. The planet topped out at 317 times the mass of the Earth.
The team reports Jupiter’s new age in the Proceedings of the National Academy of Sciences. The paper was published the week of June 12.
“I have high confidence that their data is excellent,” says Meenakshi Wadhwa. She works at Arizona State University in Tempe. She is a cosmochemist. That means she studies the chemistry of the matter in the universe. The suggestion that Jupiter held the different groups of space rocks apart is “a little more speculative, but I buy it,” she adds.
Jupiter’s early birth could also explain why the inner solar system lacks any planets larger than Earth. Many planetary systems far beyond the sun have large, close-in planets. These can be rocky planets a bit bigger than Earth, known as super-Earths. They are about two to 10 times the mass of Earth. Or, there can be gassy mini-Neptunes or hot Jupiters.
Astronomers have puzzled over why our solar system looks so different. If Jupiter formed early, its gravity could have kept most of the planet-forming disk away from the sun. That means there was less raw material for the inner planets. This picture is consistent with other work. That research suggests a young Jupiter wandered through the inner solar system and swept it clean, Kruijer says.
“Without Jupiter, we could have had Neptune where Earth is,” Kruijer says. “And if that’s the case, there would probably be no Earth.”
acid A chemical that releases hydrogen ions when dissolved in a solution. Acids have a sour taste and have a pH ranking of less than 7.0.
asteroid A rocky object in orbit around the sun. Most orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.
chemistry The field of science that deals with the composition, structure and properties of substances and how they interact with one another. Chemists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) The term is used to refer to the recipe of a compound, the way it’s produced or some of its properties.
cosmochemistry The field of science that deals with the composition, structure and properties of substances of the universe. Cosmochemists study the composition, structure and substances of matter in the universe.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
geochemistry A science that deals with the chemical composition of and chemical changes in the solid material of Earth or of another celestial body (such as the moon or Mars). Scientists who study geochemistry are known as geochemists.
gravity The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.
isotopes Different forms of an element that vary somewhat in weight (and potentially in lifetime). All have the same number of protons but different numbers neutrons in their nucleus. As a result, they also differ in mass.
Jupiter (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
meteor A lump of rock or metal from space that hits the atmosphere of Earth. In space it is known as a meteoroid. When you see it in the sky it is a meteor. And when it hits the ground it is called a meteorite.
Neptune The furthest planet from the sun in our solar system. It is the fourth largest planet in the solar system.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now includes eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
planetary science The science of other planets besides Earth.
solar system The eight major planets and their moons in orbit around the sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.
solution A liquid in which one chemical has been dissolved into another.
strong acid This is an acid that forms when a starting material completely ionizes — each of its molecules donates a proton to a water molecule — in an aqueous solution. Examples of strong acids include hydrochloric acid. This develops when hydrogen chloride ionizes in water to form the hydrochloric acid. Other strong acids include nitric, sulfuric and perchloric. Usually, the water molecules can easily give up the protons to make the starting material again. But in strong acids, this does not generally occur. Strength does not refer to how concentrated or how corrosive an acid is.
super-Earth A planet (in a distant solar system) with between one and 10 times the mass of Earth. Our solar system contains no super-Earths: All of the other rocky planets (Mercury, Venus, Mars) are smaller and less massive than Earth, and the gas giants (Jupiter and Saturn) and ice giants (Neptune and Uranus) are all larger, containing at least 14 times the mass of Earth.
universe The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).
Journal: T.S. Kruijer et al. Age of Jupiter inferred from the distinct genetics and formation times of meteorites. Proceedings of the National Academy of Sciences. Published online the week of June 12, 2017. doi: 10.1073/pnas.1704461114.